Reformist group Bersih, whose colour is yellow, turned out in huge numbers for the second time in 15 months despite a police ban on the rally and the arrest of 17 activists so far.

Tens of thousands of anti-government protesters, clad in yellow t-shirts, gathered in the Malaysian capital on Saturday seeking Prime Minister Najib Razak’s resignation over graft allegations. Reformist group Bersih, whose colour is yellow, turned out in huge numbers for the second time in 15 months despite a police ban on the rally and the arrest of 17 activists so far.

Deputy Prime minister Ahmad Zahid Hamidi, who is also the Home Minister, warned more protesters could be arrested.

Police blocked at least 60 roads across the city and put up barricades at entry points in downtown Kuala Lumpur. Water canon trucks and security personnel have been deployed around the downtown areas.

The Bersih tweeted that police raided its office on Friday and detained its chairwoman Maria Chin.

“Despite the authorities’ desperate measures to stop us, (the rally) will go on,” the group tweeted.

Ruling party politician Jamal Yunos, who planned to lead a counter rally, also was detained.

Authorities had banned the holding of both rallies – by anti-government and pro-government supporters.

Prime Minister Najib, who is attending an Asia-Pacific summit in Peru, has remained unmoved by reports of graft allegations against him involving the indebted 1 Malaysia Development Berhad (1 MDB) state fund that he founded.

1 MDB is at the centre of investigations in the US and several other countries.

In a statement on his blog, Najib called Bersih “deceitful” and said it has become a tool for opposition parties to unseat a democratically elected government.

Human rights group Amnesty International called for the immediate release of the Bersih activists.

In his biggest political crisis since he took power in 2009, massive street rallies were held against Najib in August calling for his resignation after documents leaked in July suggested that about USD 681 million was deposited in his private bank accounts from entities linked to indebted state investment fund 1 MDB.